1. Technical Field
The present disclosure generally relates to photoreceptors and methods for fabricating photoreceptors. More particularly, the disclosure generally relates to photoreceptor device surfaces and fabrication methods that suppress a “plywood effect”.
2. Description of Related Art
Substrates used for photoreceptors are typically made from cylindrical aluminum tubes. To achieve the desired dimensional properties required for these devices the aluminum tubes are often machined on a lathe and left with a specular or mirror surface which produces congruent reflection upon exposure to radiation. When employing coherent exposure radiation for printer products an undesirable print artifact termed “plywood” is produced in the high density areas as described in further detail hereinbelow.
There are numerous applications in the electrophotographic art wherein a coherent beam of radiation, typically from a helium-neon or diode laser is modulated by an input image data signal. The modulated beam is directed (scanned) across the surface of a photosensitive medium. The medium can be, for example, a photoreceptor drum or belt in a xerographic printer, a photosensor CCD array, or a photosensitive film. Certain classes of photosensitive medium which can be characterized as “layered photoreceptors” have at least a partially transparent photosensitive layer overlying a conductive ground plane. A problem inherent in using these layered photoreceptors, depending upon the physical characteristics, is the creation of two dominant reflections of the incident coherent light on the surface of the photoreceptor, for example, a first reflection from the top surface and a second reflection from the bottom surface of the relatively opaque conductive ground plane. This condition is shown in FIG. 1; coherent beams 1 and 2 are incident on a layered photoreceptor 6 comprising a charge transport layer 7, charge generator layer 8, and a ground plane 9. The two dominant reflections are: from the top surface of layer 7, and from the top surface of ground plane 9. Depending on the optical path difference as determined by the thickness and index of refraction of layer 7, beams 1 and 2 can interfere constructively or destructively when they combine to form beam 3. When the additional optical path traveled by beam 1 (dashed rays) is an integer multiple of the wavelength of the light, constructive interference occurs, more light is reflected from the top of charge transport layer 7 and hence, less light is absorbed by charge generator layer 8. The difference in absorption in the charge generator layer 8, typically due to layer thickness variations within the charge transport layer 7, is equivalent to a spatial variation in exposure on the surface. This spatial exposure variation present in the image formed on the photoreceptor becomes manifest in the output copy derived from the exposed photoreceptor. FIG. 2 shows the areas of spatial exposure variation (at 25×) within a photoreceptor of the type shown in FIG. 1 when illuminated by a He—Ne laser with an output wavelength of 633 nm. The pattern of light and dark interference fringes look like the grains on a sheet of plywood. Hence the term “plywood effect” is generically applied to this problem.
One method of compensating for the plywood effect known to the prior art is to increase the thickness of and, hence, the absorption of the light by the charge generator layer. For most systems, this leads to unacceptable tradeoffs; for example, for a layered organic photoreceptor, an increase in dark decay characteristics and electrical cyclic instability may occur. Another method, disclosed in U.S. Pat. No. 4,618,552 is to use a photoconductive imaging member in which the ground plane, is formed with a rough surface morphology to diffusively reflect the light.
U.S. Pat. No. 4,618,552 discloses a photoconductive imaging member in which the ground plane, or an opaque conductive layer formed above the ground plane, is formed with a rough surface morphology to diffusely reflect the light.
As discussed in the references, a method for compensating for the plywood effect is to provide for a photosensitive imaging member having a roughened surface to difftisively reflect the light. One known method for providing a roughened surface is the liquid honing technique which involves spraying the surface to be roughened with a mixture comprised of water and abrasive particles. Liquid honing, however, is disadvantageous in several respects. One disadvantage arises from the diamond turning or precision extrusion drawing of the substrate prior to liquid honing. In the diamond turning process, a diamond is utilized as a cutting tool while the substrate is rotated at high surface speed (about 20,000 inch per minute) to produce a very smooth, highly reflective surface. Typical surface finishes of about Ra≈0.05 micron and about Rt≈0.5 micron are produced. Ra represents the mean roughness of the surface, and R1 represents the vertical distance between the highest peak and the lowest valley of the roughness profile of the surface.
Typically, after diamond turning or extrusion drawing and before liquid honing, the substrate is removed from the lathe or drawing table, lubricant and/or debris resulting from the diamond turning and drawing are removed, and the substrate is cleaned and remounted on a honing machine. This procedure is inefficient since the liquid honing step cannot occur until after the substrate is remounted on the honing machine. Another disadvantage is that a liquid honed surface, such as that involving blasting aluminum substrates with abrasive particles, may exhibit a relatively irregular surface texture having angular, sharp shaped features with holes, fissures, and shaped abrasive particles which are used to hone the surface which can then impact the print quality as well as the sensitivity of the motor receptor. Honing adds to the manufacturing cost, and the resulting media can be defective, especially if it is not thoroughly cleaned after honing.
Accordingly there is a need for fabricating a photoreceptor device having an irregular surface that enables a light scattering ability to eliminate the “plywood effect”.